1. Field of the Invention
This invention relates to an electrophotographic image formation process and an apparatus for this process. More particularly, this invention relates to a color image formation process that restricts the thickness of a developer layer on a developing roller by the use of a toner layer thickness-limiting blade having a specific structure and visualizes an electrostatic latent image formed on an image support by the use of a nonmagnetic one-component developer, for example, and to an apparatus for this process. The present invention relates further to an image formation apparatus employing an electrophotographic process that uses a specific developer support and a developer feeding member in combination with a specific one-component developer.
2. Description of the Related Art
Electrophotographic image formation apparatuses such as laser printers have gained a wide application for output terminal devices of computers, facsimiles, copying machines, and so forth, with the progress of office automation. An image formation apparatus of this kind includes generally a charging device for electrically and uniformly charging a photosensitive drum as an image support, an exposing device for forming an electrostatic image on the photosensitive drum by the irradiation of light, a developing device for developing the electrostatic image on the photosensitive drum and making it visible by using a developer (toner), an image transferring device for transferring the toner image formed on the photosensitive drum by development to a recording medium such as a recording sheet, and an image fixing device for fusing the toner image so transferred to the recording medium and fixing the image to the medium.
The developing device generally comprises a developing roll so disposed as to oppose, and to come into contact with, the photosensitive drum, a toner container for storing the toner, a toner supplementing device for feeding the toner to the developing roll and a toner layer thickness-limiting blade for controlling the thickness of the toner supplied onto the developing roll. As the toner is allowed to adhere electrically and uniformly from the toner layer on the developing roll to the electrostatic latent image on the photosensitive drum, development, that is, visualization, of the electrostatic latent image, can be conducted. To use again the used photosensitive drum after the toner image is transferred, a de-charging device for removing the charge from the surface of the photosensitive drum and a cleaning device for scraping off the residual toner is disposed round the photosensitive drum.
The developing device used for the image formation apparatus described above includes a device of the type designed to use a one-component developer comprising only the toner and a device of the type designed to use a two-component developer comprising the combination of the toner and a carrier. Since the one-component type developing device does not use a carrier, it need not take into consideration degradation of the carrier, mixing of the carrier with the toner and the mixing ratio, in particular. Therefore, the one-component type developing device has the advantages that the apparatus can be made compact in size and its production cost can be lowered. Furthermore, when the developer used is nonmagnetic, this developing device can form a high-quality color image because the toner has high transparency.
When a one-component developing device is used, a process step for charging compulsively the developer, imparting the charge to the developing roller and causing the toner to adhere to the developing roller is necessary unlike the two-component developing device, that uses the developer comprising the mixture of the carrier and the toner and lets it adhere to the magnet roller, because the one-component developer used does not have a carrier. Therefore, the one-component developer uses a toner having a relatively high volume resistivity. When a toner having a volume resistivity of 1010 xcexa9cm to 1013 xcexa9cm, or more, is used, a compulsive charging operation to a predetermined polarity is necessary. Therefore, a triboelectrical or frictional charging member for imparting triboelectrical charge to the toner is also provided to the developing device.
A blade for uniformly limiting the toner adhering to the developing roller to a predetermined thickness and a charging member used for imparting exclusively triboelectrical charge to the toner, for example, have been used as the triboelectrical charging member. Among them, the blade for limiting the toner to a predetermined thickness and at the same time, charging the toner, has the simplest structure and can reduce the cost. As will be understood from the following explanation, the toner layer thickness-limiting blade used inside the developing device in the embodiments of the present invention includes a blade that has the function of exclusively limiting the toner layer thickness, a blade having the exclusive function of frictional charging, and a blade having both of these functions.
FIGS. 1 to 5 schematically depict the developing devices equipped with the conventional toner layer thickness-limiting blades (partial views).
In the developing device shown in FIG. 1, a blade 50 made of a resin or a metal having a relatively high hardness and a thickness of 2 to 4 mm is fitted into a blade guide 51 in such a fashion as to be capable of moving in and out due to a coil spring 52. The blade 50 is brought into pressure contact with a developing roller 2, rotating in a direction indicated by an arrow B, at a constant pressure. The developing roller 2 can rotate while keeping contact with an image support (typically, a photosensitive drum) 1 that is so disposed as to oppose the developing roller 2 and to be capable of rotating in a direction indicated by an arrow A.
The developing device shown in FIG. 2 uses a blade 50 produced by shaping the distal end portion of a leaf spring into an L shape. In this device, one of the ends of the blade 50 is fixed to a blade holder 51 made of a material having high rigidity, and an L-shaped edge as the other end of the blade 50 is brought into pressure contact at a constant pressure with the developing roller 2 by its own flexibility.
In the developing device shown in FIG. 3, a blade 50 made of a flexible material such as a rubber is bonded to, and extended from, one of the ends of the blade holder 51, and the distal end portion of the blade 50 is brought into pressure contact with the developing roller 2.
The developing device shown in FIG. 4 uses a blade 50 formed by shaping the distal end portion of a leaf spring into a U-shaped. In this device, one of the ends of the blade 50 is fixed to a blade holder 51 made of a material having high rigidity, and a U-shaped surface as the other end of the blade 50 is brought into pressure contact with the developing roller 2 at a predetermined pressure by its own flexibility.
In the developing device shown in FIG. 5, one of the ends of a blade 50 comprising a leaf spring is fixed to a blade holder 51. The distal end of the blade is subjected to rounded edge machining to impart roundness (not shown). The edge portion having this roundness is brought into pressure contact with the developing roller 2 at a constant pressure.
However, the toner layer thickness-limiting blades used in the developing devices shown in FIGS. 1 to 5 involve respective problems to be solved. The toner layer thickness-limiting blade shown in FIG. 1, for example, involves the problems of distortion of the developing roller resulting from creep, the occurrence of horizontal stripes resulting from this distortion and the occurrence of xe2x80x9cfogxe2x80x9d resulting from non-uniformity of the toner layer thickness. The blade shown in FIG. 2 involves the problem of deterioration of the toner resulting from fine cracks at the L-shaped edge. The blade shown in FIG. 3 involves the problem of the drop of the frictional charging capacity resulting from creep. The blade shown in FIG. 4 involves the problem of fixation of the toner resulting from a limit to planarity. Furthermore, the blade shown in FIG. 5 involves the problem of non-uniformity of the toner layer thickness resulting from a limit to planarity and the occurrence of xe2x80x9cfogxe2x80x9d resulting from the former.
These problems are particularly serious when a nonmagnetic one-component developer is used. When such a developer is used, the toner layer thickness-limiting blade must be able to come into uniform pressure contact with the developing roller at a constant pressure, to uniformly limit the toner thickness to a predetermined thickness and to uniformly charge the toner without inviting deterioration of the toner.
The resolution required of the one-component developer has increased year by year, in digital copying machines and printers, and the requirement for toners having smaller particle sizes has become stronger. Recently, toners having small particle diameters, the weight mean particle diameters of which fall within the range of about 6.0 to 10.0 xcexcm, have been frequently used in these apparatuses. Furthermore, toners that can be fixed even at a low temperature have been required to cope with the energy saving trend of the apparatuses, and the thermal characteristics of the toners have been shifted to the lower temperature side with the requirement for color printing.
Under such circumstances, the following problem develops when a xe2x80x9ctoner having a weight average particle diameter of 6.0 to 10.0 xcexcm and low thermal characteristics (that is, fixable at a low temperature)xe2x80x9d is used in the developing devices explained above with reference to FIGS. 1 to 5. As the developing roller is rotated for a long time while the blade is kept in pressure contact with the developing roller, the toner receives thermal/mechanical stress when it passes under the blade and, consequently, the toner is fused at the distal end of the blade as printing is repeated. As a result, stable formation of the toner layer is impeded on the photosensitive drum and white stripes occur and deteriorate image quality.
As described above, in the image formation apparatus that forms an electrostatic image on an image support by the electrophotographic process and develops it by using a developer to a visible image, a developing device using a one-component developer is more advantageous from the aspects of the size and cost of the device and reliability. To form a color image, in particular, a nonmagnetic one-component developer is advantageous because it has high transparency.
Developing devices of various types use the nonmagnetic one-component developer. A typical developing device includes a developer support that supports the one-component developer on its surface and transfers it along a predetermined circulation path inclusive of a developing region, storing means for storing the one-component developer and developer feeding means coming into contact with the developer support, for feeding the one-component developer stored in the developer storing means to the developer support. Such a developing device is described in detail in, for example, Japanese Unexamined Patent Publications (Kokai) No. 60-229057 and No. 61-42672.
FIG. 6 schematically shows an example of the developing device described above. The developing device 110 is equipped with a casing 113 for defining a developer container (toner hopper) that stores a nonmagnetic one-component developer not containing a magnetic material and comprising only a toner, that is, a nonmagnetic toner 111, and includes, inside this casing 113, a developing roller 114, a sponge roller 115 for supplying the developer to the developing roller 114, and a thickness limiting blade 116 for limiting the thickness of the developer on the surface of the developing roller 114. A suitable developing bias voltage can be applied from a bias power source 121 to the developing roller 114.
Fine silica powder, for example, is added as an additive to the nonmagnetic toner 111. Fine silica powder has the function of controlling the frictional charge quantity of the toner 111 and can contribute to the improvement of the image density. The developing roller 114 is so disposed as to oppose and to come into contact with a photosensitive drum 101 that forms an electrostatic latent image, at the opening of the casing 113 and holding it. The developing roller 114 rotates in the same direction as the photosensitive drum 101 at its opposed portion with the latter. In consequence, the developing roller can transfer the toner 111 supported on the developing roller 114 to the photosensitive drum 101.
The sponge roller 115 is made of a sponge material having flexibility. The sponge roller 115 comes into flexible contact with the developing roller 114 on the opposite side to the photosensitive drum 101, rotates in the opposite direction (so-called xe2x80x9ccounter-rotationxe2x80x9d) at the contact portion with the developing roller 114, and can simultaneously scrape off the residual toner of development (the toner that is not transferred to the photosensitive drum, hence, is not used for development) and can supply the new toner 111 to the developing roller 114 inside the casing 113. The toner 11 supplied afresh at the sponge roller 115 undergoes friction due to the developing roller 114 and the sponge roller 115, and acquires the charge due to frictional charging, is allowed to adhere to the developing roller 114 by the image force and is transferred. On the other hand, the residual toner of development is scraped-off by the nip generated by the mechanical frictional force between the developing roller 114 and the sponge roller 115. When charging and the supply of the new toner are carried out simultaneously with scraping-off of the residual toner of development in this way, the nip width between the sponge roller 115 and the developing roller 114 is preferably as large as possible to sufficiently obtain these functions. To charge the toner and to scrape off the residual toner, the nip pressure is preferably high. When rotary members such as the sponge roller and the developing roller are used in combination, the effect of substantially increasing the nip width can be obtained when the linear velocity due to the counter rotation is greater. In the conventional developing devices of this kind, therefore, it has been customary to stipulate the hardness of the sponge roller or the nip width with the developing roller (for example, Japanese Unexamined Patent Publication (Kokai) No. 7-44023), or to set the linear velocity of the sponge roller to a higher level than the linear velocity of the developing roller.
The thickness-limiting blade 116 is fitted above the developing roller 114 inside the casing 113 and is brought into contact with the peripheral surface of the developing roller 114 in such a fashion as to be capable of counter-rotating with respect to the developing roller 114. Therefore, the thickness-limiting drum frictionally charges the toner 111 during its transfer to the photosensitive drum 101, and the toner thus acquires the frictional charge. To effectively impart this frictional charge to the toner 111, a member that is charged in the opposite polarity to the charge polarity of the toner 111 is disposed, in some cases, on the contact surface of the thickness-limiting blade 116 with the developing roller 114. The toner 111 transferred to the developing region on the photosensitive drum 101 is used for developing the electrostatic latent image that has already been formed in this region.
The conventional image formation apparatuses using the developing device of this kind, however, employ a construction wherein the sponge roller 115 executes counter-rotation with a large nip width and a high nip pressure with respect to the developing roller 114, and has a higher linear velocity than that of the developing roller. Therefore, the following problems develop.
1) Mechanical torque becomes high.
2) Mechanical stress on the toner increases, and deterioration of image quality is accelerated.
3) The toner supply quantity to the sponge roller becomes excessive, and the density of the lastly printing portion increases.
In view of the facts described above, these problems can be solved by reducing the nip width of the sponge roller 115 relative to the developing roller 114 and the nip pressure, and by further bringing the linear velocity of the sponge roller 115 close, or equal, to the linear velocity of the developing roller 114.
However, when the linear velocity of the sponge roller 115 is brought close, or equal, to the linear velocity of the developing roller 114, the following new problems arise.
4) Since the quantity of the toner supplied from the sponge roller to the developing roller becomes insufficient, a negative after-image occurs in the sponge roller cycle.
5) Since the frictional charge of the toner is insufficient, non-uniformity in the charge quantity of the toner, deterioration of image quality such as photographic fog of the background, deterioration of development and transfer, and a drop in resolution occur.
6) Since selective development of the toner occurs, a positive after-image occurs in the developing roller cycle.
To begin with, the negative after-image in the sponge roller cycle will be explained. The sponge roller 115 has the function of supplying the toner to the developing roller 114 as described above. The sponge roller 115 can supply a new toner to the positions of the developing roller 114 at which the toner that develops the latent image is lost. After supplying the toner, the sponge roller 115 transfers the new toner from the developer container (toner hopper) defined by the casing 113 and prepares again for supplying the toner. At this time, the difference of the toner quantity that can be supplied to the developing roller 14 occurs at the portion at which the toner is once supplied and the portion at which it is not yet supplied. Insufficiency of the toner occurs at the portion at which the toner is once supplied. Such a difference of the toner quantity results in the difference of density in the resulting toner image. This phenomenon represents the term xe2x80x9cnegative after-image in sponge roller cyclexe2x80x9d as used in this specification. The negative after-image in the sponge roller cycle appears generally at the position of one turn of developing roller+one turn of sponge roller. Particularly, the printing after-image in the solid patch printing that have a large toner consumption quantity is remarkable.
Next, deterioration of image quality resulting from non-uniformity of the toner charge quantity will be explained. When the toner does not undergo sufficient frictional charging between the sponge roller 115 and the having a small particle size, and the toners having a large toner remain inside the toner hopper. When the toner on the developing roller 114 repeats the image formation that needs a small toner consumption quantity such as white solid printing, the toner selection function operates whenever the toner passes through the nip between the developing roller 114 and the sponge roller 115. Consequently, the particle size of the toner on the developing roller becomes smaller and smaller. Because the toner having a high charge adheres strongly to the developing roller, the potential of the toner layer becomes high. When the developing roller executes printing with high toner consumption such as black solid patch printing, a new toner is supplied to the position at which the toner is consumed. However, this toner has a greater toner particle diameter and a smaller charge quantity than the toner at portions at which printing does not exist. Therefore, the toner layer at this portion has obviously a different condition from the condition of the neighboring toner layers, and generates the phenomenon in which printing becomes dense (positive after-image). The selective toner feed phenomenon generates the positive after-image in this way. A positive after-image is a phenomenon that is generated because the toner is not a single substance but has a distribution of chargeability. This chargeability depends mainly on the size of the particle diameter. Since the selective toner feed phenomenon is the one that results from the difference of chargeability, it is likely to occur remarkably under the developing condition described above where satisfactory charging is not made, hence, the positive after-image, too, is likely to occur.
The present invention is directed to solve the problems of the prior art technologies described above.
It is an object of the present invention to provide an image formation method that can use a toner layer developing roller 114, the toner supplied to the developing roller 115 does not reach the saturation charge quantity. Consequently, the toner contains the toner that is hardly charged or is not at all charged, or the toner that is oppositely charged. Then, the toner having these inappropriate charge quantities hinders development transfer to the latent image with fidelity, and causes photographic fog of the base due to adhesion to the background portion or deterioration of resolution such as the failure of a delicate expression using repeated separate many dots. These problems can be solved when the toner supplied undergoes complete saturation charge.
Next, the positive after-image in the developing roller cycle will be explained. A positive after-image is a phenomenon in which the printing density of the position at which printing is made becomes high, contrary to the negative after-image. The positive after-image occurs in the rotating cycle of the developing roller. The mechanism of the occurrence of the positive after-image is closely associated with the development mechanism by the nonmagnetic one-component developing method. The toner supplied from the sponge roller 115 adheres to the developing roller 114 by the image force due to its charge, as described above. Therefore, the adhesion force is proportional to the magnitude of the charge. The greater the charge, the more likely is the toner to adhere to the developing roller 114, and the smaller the charge, the more difficult it is for the toner to adhere to the developing roller 114. As a result, the toner is selectively supplied to the developing roller 114 depending on the charge quantity (that is, the selective toner feeding phenomenon). According to an investigation done by the present inventors, the toner having a high charge quantity is the toner that has a small particle diameter. The toners are supplied selectively from the toners thickness-limiting blade capable of exhibiting the function of the layer thickness limitation or triboelectrical charging, or both of them, in combination with a toner having a weight average particle diameter of 6.0 to 10.0 xcexcm and low heat characteristics, is hence simple in construction and can provide high image quality and, moreover, high reliability.
It is another object of the present invention to provide an image formation apparatus that will be suitable for executing the image formation method of the present invention.
It is still another object of the present invention to provide an image formation apparatus equipped with a developing device that can prevent the occurrence of the negative after-image of a sponge roller cycle resulting from an insufficient amount of the toner supplied from a sponge roller, background fog due to non-uniform toner charge amount resulting from insufficiency of triboelectrical charge of the toner, deterioration of development transfer and resolution and, furthermore, the positive after-image of a developing roller cycle resulting from selective development of the toner, without inviting deterioration of image quality resulting from toner deterioration and a mechanical torque, while maintaining long-term stability.
It is still another object of the present invention to provide a color image formation apparatus for visualizing colors transmitting through the toner such as color images and, eventually, a color image formation apparatus using the toner the image density of which does not get into saturation with respect to its adhesion amount, that is, the toner the melt-viscosity of which is limited to a certain range, at the time of fixing. When such a toner is used, the melt-viscosity at the time of fixing is appropriate, and the smoothness of the image can be improved. Consequently, a high quality image having luster can be formed.
The above and other objects of the present invention will be appreciated more clearly from the description set forth below with regard to the preferred embodiments thereof.
According to one aspect of the present invention, there is provided an image formation method for forming a color image by using a contact type nonmagnetic one-component developing device in accordance with an electrophotographic process, characterized in that, inside the developing device, a flat sheet-like blade made of a metal flexible member, and having a distal end thereof chamfered, is used as a toner layer thickness-limiting blade, and a toner having a glass transition point (Tg) of 55 to 70xc2x0 C., and a weight average particle diameter of 6.0 to 10.0 xcexcm, and containing up to 20 number % of particles having particle diameters of 5 xcexcm or below, is used as a developer.
According to another object of the present invention, there is provided an image formation apparatus for forming a color image, including a contact type nonmagnetic one-component developing device, characterized in that the developing device has a flat sheet-like blade made of a metal flexible member and having a distal end portion thereof chamfered, as a toner layer thickness-limiting blade for limiting a developer layer thickness on a developing roller provide thereto, and a toner having a glass transition point (Tg) of 55 to 70xc2x0 C. and a weight average particle diameter of 6.0 to 10.0 xcexcm, and containing up to 20 number % of particles having particle diameters of 5 xcexcm or below, is used as the developer stored in the developing device. Hereinafter, this image formation device is called the xe2x80x9cfirst image formation apparatusxe2x80x9d.
According to still another aspect of the present invention, there is provided an image formation apparatus for forming an image by visualizing an electrostatic latent image by using a developer, including a developing device including a developer container for storing a one-component developer; an image support for forming and holding an electrostatic latent image; a developer support for transferring the developer to a developing region on the image support, disposed opposite to the image support while keeping contact with the image support; a developer feeding member for supplying the developer inside the developer container to the developer support, disposed to be capable of moving while keeping flexible contact with the developer support; and a thickness-limiting member for limiting the thickness of the developer on the developer support, supplied from the developer feeding member; wherein: the developer support is a rotary member having an outer diameter Dd and a surface linear velocity Vd, the developer feeding member is a rotary member having an outer diameter Dr and a surface linear velocity Vr, and the developer support and the developer feeding member satisfy the relation Ddxe2x89xa7Dr and Vd=Vr; and wherein the one-component developer comprises particles having a weight average particle diameter of 6.0 to 10.0 xcexcm, and containing 0 to 20 number % of particles having particle diameters of 5 xcexcm or below and 0 to 2 vol % of particles having a volume average particle diameter of 12.7 xcexcm or above. This image formation device will sometimes be called the xe2x80x9csecond image formation apparatusxe2x80x9d.
The present invention uses the developing device in which the linear velocity of a developer feeding member (typically, the sponge roller, as will be explained later) is set to an equal speed to the linear velocity of a developer support (typically, a developing roller), and the outer diameter of the sponge roller is smaller than the outer diameter of the development roller. Therefore, the present invention can extend the life of the image formation apparatus. At the same time, the present invention uses a toner having a specific particle size distribution. In other words, the volume average particle diameter of the toner particles is within the range of 6 to 10 xcexcm, the proportion of particles having particle diameters of 5 xcexcm or below is within the range of 0 to 20 number %, and the proportion of particles having particle diameters of 5 xcexcm or below is 0 to 20 vol %. In this way, the present invention can prevent particle diameter selection, that is, the phenomenon in which the toners having smaller particle diameters are supplied preferentially between the sponge roller and the developing roller. Even when this particle diameter selection occurs to a certain extent, the present invention can minimize the particle diameter shift of the toner remaining inside a developer container (typically, a toner hopper). When the image formation apparatus according to the present invention is used, therefore, the occurrence of the positive after-image of the developing roller cycle can eventually be prevented.
According to still another aspect of the present invention, there is provided an image formation apparatus for forming an image by visualizing an electrostatic latent image by using a developer, including a developing device and a developer container for storing a one-component developer; an image support for forming and holding an electrostatic latent image; a developer support for transferring the developer to a developing region on the image support, so disposed as to oppose the image support while keeping contact with the image support; a developer feeding member for supplying the developer inside the developer container to the developer support, so disposed as to be capable of moving while keeping flexible contact with the developer support; and a thickness-limiting member for limiting the thickness of the developer on the developer support, supplied from the developer feeding member; wherein: the developer support is made of an electrically conductive material and its electric resistance Rd is 1xc3x97103 to 1xc3x97108 xcexa9; and the developer feeding member is made of an electrically conductive material, and its electric resistance Rr satisfies the relation:
xe2x88x924 less than log(Rd/Rr)xe2x89xa64(log(Rd/Rr)xe2x89xa00).
Hereinafter, this image formation apparatus will be called the xe2x80x9cthird image formation apparatusxe2x80x9d.
In the third image formation apparatus, the developing roller assembled into the developing device is made of an electric conductor, and its electric resistance Rd is 1xc3x97103 to 1xc3x97108 xcexa9. The sponge roller for supplying the toner is also made of a conductor and its electric resistance Rr is set to xe2x88x924xe2x89xa6log(Rd/Rr)xe2x89xa64(log(Rd/Rr)xe2x89xa00). In this way, the positive after-image of the developing roller cycle can be prevented. The smaller the difference of the electric resistance between Rd and Rr in this image formation apparatus, the more easy it becomes to restrict an unnecessary current applied to the toner. Therefore, when this image formation apparatus is used, the toner can catch the charge by only the charge of pure charge of friction, and non-uniformity of the charge held by the toner decreases. Therefore, the selective supply phenomenon of the toner can be controlled.
In the second and third image formation apparatuses according to the present invention, the charge amount of the one-component developer on the developer support is preferably within the range of xe2x88x9240 to xe2x88x9260 xcexcC/g. When such a charge amount is adopted, the negative after-image of the reset roller cycle and the positive after-image of the developing roller cycle can be prevented.
The charge amount of the one-component developer is preferably within the range of xe2x88x9230 to xe2x88x9250 xcexcC/g within a predetermined time (Drxc3x97xcfx80/Vr). In other words, in the image formation apparatus of the present invention, the negative after-image of the reset roller cycle can be prevented when the charge amount of the toner on the developing roller is allowed to increase to xe2x88x9230 to xe2x88x9250 xcexcC/g within the predetermined period. The inventors of the present invention have discovered that it is one of the most important factors for preventing the after-image that the charge amount reaches a suitable charge amount at the developing start time and that this suitable charge amount is maintained.
The one-component developer used for the image formation apparatus according to the present invention preferably has a melt-viscosity of 50,000 Paxc2x7sec or below at 100xc2x0 C.